Article: Octane: the facts and the fiction

June 10, 2018

Tech 101 – Octane: the facts and the fiction behind those higher-priced fuels

Jim O'Clair on Apr 11th, 2014 Photo Demond Henderson.

Any discussion about octane invariably leads to statements from some cars’ owners that their engine performs better when they use the 91 or 93 (or higher) fuel blends in their vehicles.

For most modern, computer-controlled cars on the road today, this perception is more mental than it is factual. For classic car owners, octane can make a difference from an engine-efficiency standpoint; however, the octane rating of your gasoline has very little to do with the horsepower or torque output of your classic engine as is often alluded to in these conversations.

Octane is simply a measure of the fuel makeup, and its tendency or resistance to cause engine knock or ping when used under duress (higher RPM). The octane index rating is not based on a quantity of a chemical in the fuel mixture, but is a measure of the efficiency of the fuel blend, expressed as a ratio, relative to the efficiency of a pure hydrocarbon, which would have an octane index rating of 100 (or 100 percent). Because gasoline is made up of many different hydrocarbons, the octane rating is a comparison of the anti-knock characteristics of the blend relative to the anti-knock characteristics of a pure hydrocarbon with a 100 percent rating. Aircraft or racing fuels have a rating above 100 because the additives in the fuel raise the efficiency beyond that of a pure hydrocarbon.

Click here to go to Hemmings Daily and read Jim's complete article.

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